1. Field of the Invention
This invention relates to an anti-skid control system for achieving a satisfactory rotational behavior of the wheels of a motor vehicle during a braking operation. More particularly, this invention pertains to an anti-skid control system which is designed to increase and reduce brake fluid pressure in a desirable manner to prevent the wheels from becoming locked.
Especially, the present invention relates to means for avoiding malfunctions of the anti-skid braking system by changing the factors for controlling the braking system in accordance with the velocity of the vehicle.
2. Description of the Related Art
There have been heretofore proposed a variety of anti-skid control systems wherein, during a braking operation, the brake fluid pressure is reduced to prevent the occurrence of an undesirable wheel lock, and the brake fluid pressure thus reduced is then increased to avoid an undesirable extension of the brake stopping distance. Among such conventional anti-skid control systems in one wherein different pressure buildup rates are stored; a desired one of the stored pressure buildup rates is selected; and brake fluid pressure is increased with the selected buildup rate, for example. To effect the pressure buildup rate selection, a system has been proposed, wherein a solenoid-operated valve adapted to be opened and closed with a relatively high frequency is provided in the brake fluid pressure buildup system; a pulse train generator such as multivibrator is provided in the drive system for the solenoid-operated valve; and the timing with which the solenoid-operated valve is driven by a pulse train signal derived from the multivibrator is controlled on the basis of a signal representing wheel acceleration (British Patent Specification No. 1305430).
Conventionally, the applicants have been developing a wheel speed sensor which generates a pulse signal having pulses the cycle of which corresponds to the revolutional speed of the wheel. The cycle of the pulse signal is measured by counting the number of clock pulses included within the cycle, as described in detail later. In this measuring method, as illustrated in FIG. 4, an error of one clock pulse occurs in accordance with the relative position between the wheel speed pulse signal and the clock pulse signal. The cycle of the wheel speed pulse signal becomes short when the wheel velocity becomes fast. FIG. 5 shows relationship between the wheel velocity V and the cycle T of the wheel speed pulse signal generated from the wheel speed sensor. In accordance with the increase of the velocity V, the cycle T becomes short, thus the difference of one clock pulse affects the detected velocity more significantly in the high speed region than in the low speed region (V.sub.L &gt;V.sub.S). Especially, when the vehicle speed exceeds 110 km/h, the adverse influence of the error becomes so great that the anti-skid brake system malfunctions if the system is controlled in accordance with the same controlling factors as in the low speed region.